Big 7-segment countdown timer

The main challenge was driving the LEDs with a constant current, because they need about 10.5 volts across the several series LEDs. I wanted to run from 12 volts, so there wasn’t much voltage left over for the normal current limiting resistors. Instead, I used this opamp circuit.

Comments

This circuit is a nice example of how op-amp feedback can be used to control a transistor that’s handling far more current than the op-amp could muster on its own. However, that’s a lot of op-amps!

A simpler schematic would replace each op-amp with a diode across the base-emitter junction, such that a resistor can be used to set the current. This is like the revised LED driver in the newer USB Infrared Toy v2. The $0.23 quad op-amp can be replaced by three or four $0.04 MM4148 diodes. Some of the other op-amp support passives could be removed, as well.

Of course, that’s just the output section of this project. The Teensy could be totally removed by using BCD counter chips that have increment-decrement input pins. Rather than having 4 serial-to-parallel chips and a processor, the same number of chips can be used that are basically pre-programmed for this function. I’m talking about old-school discrete logic (well, not really that discrete, when we’re talking about BCD counters). The advantage would be that you don’t have to load code into the board to get it to work.